1. Field of the Invention
The present invention generally relates to a process for setting up a Radio Resource Control (RRC) connection in a mobile communication system. More particularly, the present invention relates to a method and apparatus for receiving a response message for an RRC connection request message.
2. Description of the Related Art
FIG. 1 illustrates the configuration of a 3rd Generation Partnership Project (3GPP) Universal Mobile Telecommunication System (UMTS) network.
Referring to FIG. 1, the UMTS system includes a Core Network (CN) 100 and a plurality of Radio Network Subsystems (RNSs) 110 and 120.
The RNSs 110 and 120 form a UMTS Terrestrial Radio Access Network (UTRAN). The CN 100 includes a Mobile Switching Center (MSC) and a Visitor Location Register (VLR) to connect the UTRAN to a circuit network for a Circuit-Switched (CS) call. The CN 100 further includes a Serving General packet Radio Service (GPRS) Support Node (SGSN) and a Gateway GPRS Support Node (GGSN) to connect the UTRAN to a packet data network such as the Internet.
Each of the RNSs 110 and 120 includes a Radio Network Controller (RNC) 311 or 312 and a plurality of Node Bs 113 and 115 or 114 and 116. Specifically, the RNS 110 has the RNC 111 and the Node Bs 113 and 115 and the RNS 120 has the RNC 112 and the Node Bs 114 and 116.
The RNCs 111 and 112 are classified as a serving RNC, a drift RNC, or a control RNC according to their roles. The serving RNC manages information about each User Equipment (UE). The serving RNC is responsible for CS calls and packet transmission with the CN 100. The drift RNC is wirelessly connected directly to a UE. The control RNC controls radio resources of each Node B.
The RNCs 111 and 112 are connected to the Node Bs 113 to 116 by interfaces called Iubs. The RNCs 111 and 112 are connected to each other via an Iur interface. While not shown, a UE 130 is connected to the UTRAN via a Uu interface.
The RNCs 111 and 112 each allocate radio resources to the Node Bs 113 to 116 that the RNCs manage and the Node Bs 113 to 116 actually provide the allocated radio resources to the UE 130. The radio resources of each Node B are those for a particular cell that the Node B manages. The UE 130 establishes a radio channel using the radio resources of a particular cell that the Node B 113, 114, 115 or 116 manages and exchanges data on the radio channel.
FIG. 2 illustrates a control plane architecture for the 3GPP system.
Referring to FIG. 2, reference numeral 201 denotes a UE protocol stack and reference numeral 211 denotes a Radio Access Network (RAN) stack. Reference numeral 221 denotes a CN stack. The RAN 211 is a network responsible for radio access. It includes a Node B and an RNC. The interface between the UE 201 and the RAN 211 is called a Uu interface, and the interface between the RAN 211 and the CN 221 is called an Iu interface.
The UE and the RAN send/receive radio access-associated control information through an RRC layer which usually resides in the RNC. Control information at or below the RRC layer is called Access Stratum (AS) control information. The UE and the CN exchange mobility-associated control information involving calls, sessions, and the CN through Call Control (CC), Mobility Management (MM), Session Management (SM), and Packet Mobility Management (PMM) layers. In general, the CC/MM layer is provided at the MSC and the SM/PMM layer is at the SGSN.
The CC layer controls a CS call for the UE, sends/receives UE context information and service context information for the CS call, and manages the context information. The MM layer tracks the mobility of the UE and manages the mobility, for the CS call. The SM layer controls a Packet-Switched (PS) session, sends/receives UE context information and service context information for the PS service, and manages the context information. The PMM layer tracks the mobility of the UE and manages the mobility, for the PS service. Control information between the CN and the UE at the CC, MM, SM and PMM layers above the RRC layer is called Non-Access Stratum (NAS) control information.
Since a Radio Link Control (RLC), a Media Access Control (MAC) layer, and a PHYsical (PHY) layer are beyond the scope of the present invention, they are not described herein. The functionalities of these layers comply with the 3GPP standards.
In idle mode, the UE should first perform an RRC connection establishment procedure for an RRC connection setup to initiate a CS or PS call. The idle mode is defined as a mode in which no control channel exists between the UE and the RNC. When a control channel is established between the UE and the RNC by the RRC connection establishment procedure, control information is exchanged on the control channel between the UE and the RNC, for a call setup between the UE and the RNC and between the UE and the CN, a radio bearer is established for data transmission, and data is exchanged on the radio bearer.
FIG. 3 is a diagram illustrating a signal flow for an RRC connection setup for a UE in the idle mode.
Referring to FIG. 3, reference numeral 301 denotes a UE and reference numeral 302 denotes a RAN. The UE 301 operates in the idle mode in step 311. Upon request of an upper layer for a signaling connection setup with a network node, in step 321 the UE 301 sends an RRC CONNECTION REQUEST message to the RAN 302 in order to request a control channel setup. In step 322, the RAN 302 replies with an RRC CONNECTION SETUP message on a particular downlink channel. The RRC CONNECTION SETUP message contains control channel information. For reception of the RRC CONNECTION SETUP message, the UE 301 has to listen to the downlink channel carrying the message in a current cell during a value of a timer 303 in step 331. The timer 303 is defined as T300 or T308 in the TS25.331 standard of the 3GPP UMTS. The downlink channel is a Secondary Common Control Physical Channel (SCCPCH) which is mapped using the Identifier (ID) of the UE.
Upon receipt of the RRC CONNECTION SETUP message in step 322 before the expiration of the timer 303, the UE 301 in step 323 notifies the RAN 302 of completion of the RRC establishment by an RRC CONNECTION SETUP COMPLETE message. While the RAN 302 sends the RRC CONNECTION SETUP message in response to the RRC CONNECTION SETUP REQUEST message in the illustrated case of FIG. 3, the RAN 302 may reply with an RRC CONNECTION REJECT message rejecting the RRC connection establishment.
Because of the continuous listening to the downlink channel until receiving the RRC CONNECTION SETUP message during the timer value after sending the RRC CONNECTION REQUEST message, the UE 301 does not measure the strengths of signals from inter-frequency or inter-Radio Access Technology (RAT) neighbor cells until the expiration of the timer or until receipt of the RRC CONNECTION SETUP message during the timer value. The signal strength measuring operation is referred to as “measurement”.
If the channel status of the UE becomes poor during the RRC connection establishment, the UE may not quickly reselect to a good inter-frequency or inter-RAT cell because it does not perform the measurement during the timer value.